Cleanup the rosbag2_transport tests

[clalancette]

The main goal here is to reduce compile times and memory usage. The way we do it:

1. Make as many of the dependencies private as possible. That way the main executable is exporting the minimum of dependencies to downstream consumers.
2. Compile the test files once, instead of once per RMW. We can do this because we change the RMW at runtime with an environment variable. This requires some changes to ament_cmake, which are in Split ament_add_gmock into _executable and _test. ament/ament_cmake#497

With this PR in place, build times drop by about 3x for me locally (from about 3 minutes to about 1 minute).

A draft until ament/ament_cmake#497 lands.

Fixes #872

[clalancette]

All right, ament/ament_cmake#497 is merged now. So converting this to a real PR and running CI on it.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows

[clalancette]

After a small fix here, let's try CI again:

• Linux
• Linux-aarch64
• Windows

[MichaelOrlov]

@clalancette Thanks for the PR.
Since the #872 popup again I was considering to do something similar sonner.

You mentioned that build speed increased 3x, that is a good result and could be expectable, if we now compiling for 1 RMW implementation instead of 3 + other optimizations.

But what about memory consumption during compilation? Do you see any improvements?
Sorry, I have not had a chance to experiment with it myself.
I am curious if can do even better.

[clalancette]

But what about memory consumption during compilation? Do you see any improvements?

I did not measure it during build times. I expect it will be better, just from the fact that we are compiling fewer things. In terms of improvement, I think this is the best we can do with changes to the build system only; using less memory would probably require changes to the templates, in particular. But I think that is a follow-up.

[MichaelOrlov]

@clalancette Thank you! LGTM.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows